I assume Kia calibrates the odometer for the stock size wheel for the particular trim. So if you change to a different tire size, that could affect the apparent mpg as you cannot recalibrate the speedo. However, the size difference is far smaller than you suggest if you are using the stock tire sizes. Since you are using diameter, it is 3/10 of an inch larger for the 18" tire. I doubt you could tell the impact on mpg.
In a bit of turnaround, actual mpg should increase a little with the larger tire as the final drive ratio is a bit taller. You can see that in the revs per mile figure in the attached comparison. But the drop in rolling resistance overwhelms that small advantage.
2018 Kia LX HEV Metal Stream with Advanced Tech
Last edited by yticolev; 10-27-2018 at 09:50 AM.